Many studies have shown beneficial effects of meditation. Even my dad who is about as materialist as one can get and who doesn't believe in Buddhism acknowledges the stress-reducing effect of meditation.

You can show your friend this US gov't website about it which he can use a springboard for further research as it has references. It doesn't get much more official than that.

We test three mechanisms by which the subject might stimulate his own reward system by external means and reject all three. Taken together, these results demonstrate an apparently novel method of self-stimulating a brain reward system using only internal mental processes in a highly trained subject.

Although, a lot of the research seems inconclusive, i.e.. the researches seems to suggest...Is this common in this kind of research??

Thanks.

From what little I've read, yes. I think it's hard to come to objective conclusions about the subjective "quality of life" benefits that meditation brings. And I imagine most researchers don't want to risk damaging their credibility or risk losing funding by making bold claims.